Remove stray back tick that's messing up table format (#41705)

docs/reference/cat/thread_pool.asciidoc

@@ -59,7 +59,7 @@ ml_autodetect (default distro only)
 ml_datafeed (default distro only)
 ml_utility (default distro only)
 refresh
-rollup_indexing (default distro only)`
+rollup_indexing (default distro only)
 search
 security-token-key (default distro only)
 snapshot